In the fabrication of printed wiring boards and/or printed circuit boards, soldering is the preferred technique for electrically connecting components. Soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. While both rosin and non rosin soldering fluxes are available, rosin fluxes are preferred because they are less corrosive, and have a much longer history of use. The non rosin, water soluble fluxes are a more recent development. However, these non-rosin fluxes contain strong acids and/or amine hydrohalides and are thus corrosive, and can cause circuit failure if residual traces are not carefully removed. While batch type or underbrush cleaners were initially employed, in line solvent cleaners later were adopted to meet demands of increased throughput. The removal of rosin fluxes from printed circuit boards or printed wiring boards has traditionally been accomplished using chlorinated hydrocarbon and chlorofluorinated solvents such as 1, 1, 1, -trichlorethane, trichloromonofluoromethane, methylenechloride, trichlorotrifluoroethane, methylchloroform and CFC 113.
By the early 1970's aqueous cleaners using alkaline cleaning compounds such as the alkanol amines, usually in the form of monoethanolamine, were adopted for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon and chlorofluorinated solvents. These acqueous cleaners chemically react with the rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, like the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if they are not completely and rapidly removed during the fabrication process.
With the advent of surface mounted devices which are soldered onto printed circuit boards using wave soldering or infrared solder reflow techniques, chlorinated hydrocarbon and chlorofluorinated solvents were once again required to remove the rosin fluxes because of their low cost fast evaporation characteristics, and ability to clean under lightly spaced components. However, during the past few years, it has become evident that chlorinated hydrocarbon and chlorofluorinated solvents are highly undesirable, because they are toxic, and their use is subject to close scrutiny by the Occupational Safety and Health Administration, and stringent containment equipment must be used. Moreover, if released into the environment, these solvents are not readily biodegradable, and they are thus hazardous for long periods of time. In particular, it has been discovered that chlorofluorocarbons are instrumental in depleting the ozone layer in the earth's atmosphere. This awareness of the dangers of such solvents has driven a search for safe, effective alternatives.
Alternatives which have emerged from this search are terpene compounds which have become increasingly popular in electronic cleaning applications. The use of terpene compounds for cleaning mechanical devices has been known for many years. The United States Department of Agriculture sponsored work on limonene and other citrus-based terpene compounds as long ago as the 1930's. However, there was very little interest in industrial uses of terpene compounds after that period of time, because of the availability at very low cost of halogenated solvents and petroleum distillates. With the reawakening of interest in terpene cleaners, it has been recognized that they provide a viable alternative to CFC's and other halogenated solvents for cleaning rosin fluxes from printed circuit boards. The use of one such terpene compound, which is sold under the trademark BIOACT EC 7 by Petroferm, Inc., for cleaning rosin fluxes, is the subject of U.S. Pat. No. 4,640,719 issued on Feb. 3, 1987.
In some regards, such terpene compounds are ideal for the removal of solder fluxes. Their ability to clean even the most closely spaced SMT boards has been shown in tests, and such terpene compounds work well with all available flux types. Terpene compounds are also cost competitive with most CFC's. Most terpene compounds are low in mamalian toxicity, and many are recognized as safe for use as food additives. In addition, they are produced by nearly all living plants, and therefore terpene compounds are ubiquitous components of the atmosphere for both rural and urban areas. Plants, particularly coniferous forests, release hundreds of millions of tons of terpene compounds into the air each year. In the atmosphere, terpene compounds are photochemically reactive. Wood and citrus products are the principal commercial sources of terpene compounds. Some terpene compounds are used as extracted, but most of them are obtained from natural oil refining processes. As a result, terpene compounds are commercially available in huge volumes.
Terpene compounds are readily biodegradable, and both municipal and industrial waste treatment facilities are normally able to handle terpene biodegradation without difficulty. Naturally derived terpene products contain no carbon chlorine bonds, and it is the absence of such carbon-chlorine bonds which are hard for biological systems to metabolize which renders terpene compounds so readily biodegradable. Terpene compounds can work at low temperatures, such as room temperatures, so it is not necessary to heat terpene cleaning baths. Terpene compounds are not volatile and have boiling points far above those of halogenated solvents. Also, most terpene compounds have a very high relative dissolving ability, sometimes even greater than that of any halogenated solvent.
Despite the foregoing characteristics of terpene compounds, they have yet to emerge as a commercially acceptable substitute for chlorinated and chlorofluorinated solvents. One reason is that most commercially available terpene compounds have flash points in the range of between 100.degree. F. and 200.degree. F. which causes them to be classified as combustible substances. This shortcoming must be taken into account in the production of any commercial machinery utilizing them as solvents for cleaning printed circuit boards. Another problem is the odor associated with most terpene compounds. Limonene is the terpene hydrocarbon most often used in cleaning products available in the market today. This material is derived from citrus by-products, such as oranges. The resulting odor, when concentrated, is pungent. A third problem is that, unlike chlorinated and chlorofluorinated solvents and petroleum distillates, terpene compounds are not volatile, so they will not evaporate. Therefore, they must be removed by rinsing. However, if terpene compounds are mixed with small amounts of water, they can turn to a gel and become useless. Finally, these terpene compounds tend to be very aggressive, precluding the use of certain elastomer materials in terpene cleaning systems.
It is therefore an object of the present invention to provide apparatus for the rapid, in line cleaning of printed circuit boards using terpene compounds.
It is another object of the present invention to provide apparatus for cleaning printed circuit boards which uses terpene compounds and which is safe for use with materials having flash points within the range of 100.degree. to 200.degree. F.
It is a further object of the present invention to provide apparatus for cleaning printed circuit boards which uses terpene compounds and which minimizes odor emissions to the atmosphere, and to the surrounding environment.
It is yet another further object of the present invention to provide apparatus for cleaning printed circuit boards which uses terpene compounds and which also employs a water rinse.
It is yet another further object of the present invention to provide apparatus for cleaning printed circuit boards which uses terpene compounds and which is safe for use in industrial environments.